A rare earth metal-based permanent magnet such as an R—Fe—B based permanent magnet, of which an Nd—Fe—B based permanent magnet is representative, is used at present in a variety of fields, because it has a high magnetic characteristic.
However, the rare earth metal-based permanent magnet contains metal species (in particular, R) liable to be corroded by oxidation in the atmosphere. Therefore, when the rare earth metal-based permanent magnet is used without being subjected to a surface treatment, the corrosion of the magnet is advanced from its surface due to the influence of a small amount of acid, alkali and/or water to produce rust, thereby bringing about the deterioration and dispersion of the magnetic characteristic. Further, when the magnet having the rust produced therein is incorporated into a device such as a magnetic circuit, there is a possibility that the rust is scattered to pollute surrounding parts or components.
With the foregoing in view, it is a conventional practice to form a metal deposited-film of aluminum, zinc or the like on the surface of a rare earth metal-based permanent magnet for the purpose of providing an excellent corrosion resistance to the rare earth metal-based permanent magnet.
In particular, an aluminum film is, in addition to being excellent in corrosion resistance and mass productivity, excellent in reliability of adhesion with an adhesive required in assembling parts (a peel-off is difficult to occur between the film and the adhesive before reaching an intrinsic fracture strength of the adhesive). Thus, the aluminum film is widely applied to rare earth metal-based permanent magnets for which strong adhesive strength is required. As the adhesive here, in compliance with purposes of being heat resistant, impact resistant and the like, there are selected and used various kinds of adhesives as appropriate. They are various kinds of resin adhesives such as those of epoxy resin, phenol resin, reactive acrylic resin, modified acrylic resin (ultraviolet curing adhesives and anaerobic adhesives), cyanoacrylate resin, silicone resin, polyisocyanate, vinyl acetate resin, methacrylate resin, polyamide, and polyether, emulsion adhesives of various kinds of resin adhesives (for example, vinyl acetate resin adhesives, acrylic resin adhesives and the like), various kinds of rubber adhesives (for example, nitrite rubber adhesives, polyurethane rubber adhesives and the like), and ceramic adhesives.
Examples of apparatus used for forming a metal deposited-film on the surface of a rare earth metal-based permanent magnet, include an apparatus described in U.S. Pat. No. 4,116,161 and an apparatus described in Graham Legge, “Ion Vapor Deposited Coatings for Improved Corrosion Protection”: Reprinted from Industrial Heating, September, 135-140, 1994. FIG. 1 is a diagrammatic front view (a partially perspective view) of the inside of a vacuum-treating chamber 1 connected to an evacuating system (not shown) in one example of such apparatus. Two cylindrical barrels 5, for example, formed of a mesh net of a stainless steel are disposed side-by-side in an upper area in the chamber for rotation about a rotary shaft 6 on a horizontal rotational axis. A plurality of boats 2, which are evaporating sections for evaporating, for example, aluminum as a metal depositing material, are disposed on a boat support base 4 risen on a support table 3 in a lower area in the chamber.
With this apparatus, a plurality of rare earth metal-based permanent magnets 30 as work pieces are placed into each of the cylindrical barrels 5, and aluminum is evaporated from the boats 2 heated to a predetermined temperature by a heating means (not shown), while rotating the cylindrical barrels about the rotary shaft 6, as shown by an arrow, thereby forming a deposited-film of aluminum on the surface of each of the rare earth metal-based permanent magnets 30 in the cylindrical barrels 5.
The vapor deposition apparatus shown in FIG. 1 is capable of treating a large amount of the work pieces and excellent in productivity. However, projections might have been produced in some cases on the metal deposited-film formed on the surface of each of the rare earth metal-based permanent magnets.
Presence of such projections in the film adversely affects adhesion of the magnets when assembling the magnets onto parts by using adhesive. In particular, for the projections exceeding a mean line of roughness curves (phase correct filtered mean line) according to JIS B0601-1994 by 100 μm or more, the adhesive has to be applied with a considerably large thickness so as to avoid the influence of the presence of the projections for ensuring sufficient adhesion. Therefore, use of an adhesive with low viscosity can not ensure to provide such a thickness with resulting insufficient adhesion. Moreover, use of an adhesive such as a cyanoacrylate adhesive that is cured through a chemical reaction with the surface of the film provides insufficient curing, which results in obtaining insufficient adhesion. Furthermore, even in the case where no adhesive is used as in an interior permanent magnet (IPM) type motor, presence of the projections prevents parts from being provided with high dimensional accuracy with resulting difficulty in complying with recent requirements of the parts for being reduced in size and provided with high-accuracy.
Accordingly, it is an object of the present invention to provide a method of effectively inhibiting production of projections in a metal deposited-film of aluminum, zinc or the like when forming the metal deposited-film on the surface of a work piece such as a rare earth metal-based permanent magnet.